Several catalytic processes have been developed for oligomerization of olefins to higher olefins, and in particular for oligomerization of ethylene to a series of higher olefins (C2H4)n (Equation 1).C2H4<==>C4H8, C6H12, C8H16,   (1)                i. [Catalyst]        
The higher olefins initially so formed normally are terminal(alpha) olefins ie. olefins having a single double bond at the first carbon atom. The terminal olefins may then isomerize to one or more internal olefins ie olefins having a double bond on an interior carbon atom. However, usually the terminal olefins have higher commercial utility and value than the internal olefins. For example, it is desirable to use terminal olefins in combination with ethylene to form partially branched polyolefin co-monomers, biodegradable detergents, lubricants, or plasticizers.
Thus it is desirable to operate the catalytic reaction of the process under conditions where the isomerisation reaction is minimized, thus ensuring a higher selectivity to terminal olefins. Operation at low temperatures minimizes the rate of the isomerisation reaction. However, it also is desirable to have a high reaction rate. Operation of the process at high temperatures provides a higher reaction rate than low temperatures. However, this requires high reactor pressures to allow for high olefin concentrations in the liquid phase.
There are at present three major commercial processes in use for oligomerization of olefins, each of which has a relatively high degree of complexity and less than a desirable efficiency. Both Chevron and Ethyl Corporation use Ziegler type catalysts in a homogeneous catalyst system. The Shell Higher Olefins Process (SHOP) uses a complex of nickel as the catalyst. Each of these systems uses a solvent and a catalyst in a liquid-phase reactor necessarily equipped with an intercooler. The mixture in the product stream is then purified in a series of separation columns.
Solid state catalyst processes used in slurry reactor systems allow for easier separation of the catalyst from the reaction mixtures but present several challenges. There is strong adsorption of the products on the catalyst surfaces, as well as on the reactant. Also, there is a negative thermodynamic influence on selectivity to the desired terminal olefin products at high reaction temperatures, with internal olefins being formed. There is a need for more active catalysts. Each of these factors including catalyst deactivation by the formation of decomposition and isomerisation products, must be overcome.
There are several bases for potential beneficial changes that would improve oligomerization processes, including use of milder conditions, thereby maximizing selectivity, and development of more active and selective catalysts, thereby enhancing yield and production rate.
Among the many catalysts known to catalyze the oligomerization of olefins, it has been found that highly acidic heterogeneous catalysts comprising, for example, finely divided nickel supported on sulfated alumina are particularly active for dimerization of propylene, as described in French Patent 2641 477 issued in 1990. The Ni/sulphated Al2O3 catalyst used in '477 is active at room temperature for dimerization of propylene in a slurry with an inert hydrocarbon solvent. Further, a similar catalyst comprising Ni/sulfonated non-porous Al2O3 (commercially available ALON) was shown to be active for oligomerization of ethylene, as described by Zhang et al. in “Oligomerization of Ethylene in a Slurry Reactor Using a Nickel/Sulfonated Alumina Catalyst,” Ind. Eng. Chem. Res., 36, 3433-3438 (1997), the disclosure of which is incorporated herein by reference.
Several additional processes for oligomerization of olefins have been described in patents and the open literature. Among these are descriptions of catalyst systems for oligomerization of ethylene using either homogeneous or heterogeneous catalysts. However, a characteristic of all prior art is the use of a solvent that is necessary for conducting the process, in contrast to the process of the present invention. Examples of other prior art from which the present invention is so distinguished include: Krug et al. in U.S. Pat. No. 6,841,711; Gildert et al. in U.S. Pat. 6,274,783, Vora et al. in U.S. Pat. No. 6,025,53 and Townsend et al. in U.S. Pat. No. 6,004,256.